Apparatuses and methods for cancellation of inhomogenous magnetic fields induced by non-biological materials within a patient&#39;s mouth during magnetic resonance imaging

ABSTRACT

This disclosure includes magnetic field correction devices and methods for using the same. In particular, some magnetic field corrections devices include an arch-shaped body configured to be worn outside of a user&#39;s mouth such that the arch-shaped body follows a contour of the user&#39;s face; and where the arch-shaped body comprises one or more sidewalls configured to be coupled to a plurality of members comprising magnetically permeable material. Other embodiments employ a forehead support, frame, and one or more straps coupled to an arch-shaped body. Other embodiments employ a hybrid of intraoral and external embodiments.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S.non-provisional patent application Ser. No. 14/153,516 entitled“Apparatuses and Methods for Cancellation of Inhomogeneous MagneticFields Induced By Non-Biological Materials Within A Patient's MouthDuring Magnetic Resonance Imaging,” filed on Jan. 13, 2014, whichapplication is incorporated by reference in its entirety.

FIELD OF INVENTION

The present invention relates generally to magnetic field homogenizationduring magnetic resonance imaging (MRI), and more specifically, but notby way of limitation, to apparatuses and methods for restoring losses inmagnetic field homogeneity caused by non-biological materials within apatient's mouth.

BACKGROUND

Examples of using supplementary magnetic fields to correct MRI magneticfield homogeneity are disclosed in U.S. Pat. No. 6,968,982, and Wen Z.,et. al, Shimming with Permanent Magnets for the X-Ray Detector in aHybrid X-Ray/MR System, 35(9) Med. Phys. 3895 (2008), available athttp://www.ncbi.nlm.nih.gov/pmc/articles/PMC2673662/.

During magnetic resonance imaging (MRI), magnetic fields can be inducedin non-biological materials within a patient such as medical implants ororthodontic appliances (dental braces). Non-biological materials can bemagnetized by the strong magnetic field of an MRI scanner, and theinduced magnetization in the non-biological materials can become thesource of a non-uniform magnetic field. These induced magnetic fieldscan disrupt MRI magnetic field (B₀) homogeneity and cause imageintensity losses in regions near the non-biological materials andgeometric distortions across the image. For example, during an MRI ofthe brain, non-biological materials located within the mouth of apatient may cause image intensity loss in the oral cavity and geometricdistortions over the whole brain, specifically the orbits, hypothalamus,Circle of Willis, frontal lobe, and temporal lobes. Image loss and/ordistortion is most severe for diffusion-weighted images, gradient echoimages, and magnetic resonance angiography, and there is a loss ofspectral resolution in magnetic resonance spectroscopy, as thesetechniques require a high degree of B₀ homogeneity.

Approximately 40% of the general population wears orthodontic appliancesat some point in their life, particularly during adolescence. Many, ifnot most, dental braces comprise a common non-biological material whichmay be found within a patient's mouth. More particularly, approximately95% of dental braces include brackets that comprise ferromagneticstainless steel, as stainless steel is low cost, high strength, anddurable. While orthodontic appliances, as well as other surgicalimplants, may be made of other more MRI-friendly materials such asplastic or titanium, these materials tend to be either too weak orexpensive [19-32, 43-44]. Dental braces are particularly common inchildren, causing concern for children's hospitals, since around 80% ofMRIs performed in a children's hospital involve imaging of the brain.For patients with dental braces, diffusion-weighted images and magneticresonance angiography images are generally incomprehensible andtherefore not performed. Such imaging techniques are critical fordiagnosing many serious brain conditions, such as stroke.

One method for remedying the effect of non-biological materials on MRIimaging is to remove the non-biological materials from the patientbefore performing an MRI. In the case of dental braces, this involvesremoving the patient's braces prior to performing the MRI. Often times,as is the case with dental braces, removal of the non-biologicalmaterials is time consuming, expensive, and may be unavailable inemergency or after-hours situations. Other times, the MRI scan isperformed with the non-biological materials in place, resulting insuboptimal image quality. In tumor patients requiring frequent follow-upMRI scans, removal of braces may lead to premature termination oforthodontic treatment. Current MRI technology seeks to remedy B₀inhomogeneity through a technique known as image shimming, and most ifnot all scanners (e.g., 1.5 T (1.5 tesla) scanners) are capable of shim(e.g., linear shim). However, shimming is unable to remove imageartifacts caused by non-biological materials within a patient. Otherapproaches include software correction [33-35], pulse sequence designand optimization [36-42], image unwrapping techniques [50], and sequencesegmentation [51], which may be helpful for certain types of scans.However, these techniques generally fail to directly addressinhomogeneities caused by non-biological materials within a patient(e.g., on a hardware rather than software level). While B₀ shimmingusing permanent magnets has been demonstrated in a 0.5 T X-ray/MRIhybrid system [46], permanent magnets have not been used to correctsusceptibility artifacts caused by non-biological materials within apatient.

SUMMARY

Embodiments of the present apparatuses and methods can be configured toreduce image losses and/or distortions in MRIs that would typicallyotherwise be caused by non-biological materials within a user's mouth toan acceptable level for proper diagnosis from the MRI images. Someembodiments of the present apparatuses and methods use or include aplurality of permanent magnets disposed on an apparatus configured to beplaced within a user's mouth (e.g., resembling a mouth guard) or outsideand adjacent to the a user's mouth (e.g., resembling a mouth mask),where the magnets are located near non-biological materials within theuser's mouth in an orientation such that the magnetization of themagnets opposes the MRI B₀ field when the apparatus is worn by the user.Through use of the present apparatuses, the induced inhomogeneousmagnetic field originating from the non-biological materials can besubstantially negated, leading to an increase in B₀ homogeneity andoverall improvement of MRI image quality.

Some embodiments of the present apparatuses comprise: an arch-shapedbody configured to be worn inside of a user's mouth such that thearch-shaped body follows a contour of at least some of the user's teeth;where the arch-shaped body comprises one or more sidewalls and a bitingmember, the biting member configured to be disposed between the user'smandibular and maxillary teeth, the one or more sidewalls angularlydisposed relative to the biting member and configured to be coupled to aplurality of members comprising magnetically permeable material. Someembodiments further comprise: a handle configured to protrude from theuser's mouth.

Some embodiments of the present apparatuses comprise: an arch-shapedbody configured to be worn outside of a user's mouth such that thearch-shaped body follows a contour of the user's face; where thearch-shaped body comprises one or more sidewalls configured to becoupled to a plurality of members comprising magnetically permeablematerial. The arch-shaped body may be disposed outside the user's facewithout obstructing the user's airway. For example, the arch-shaped bodymay be disposed above or below the user's mouth. In some embodiments,the plurality of members are disposed substantially adjacent to themaxilla and/or mandible of the user.

Some embodiments of the present apparatuses comprise: a hybridintraoral-external field correction device comprising a firstarch-shaped body configured to be worn inside a user's mouth such thatthe arch-shaped body follows a contour of at least some of the user'steeth; where the arch-shaped body comprises one or more sidewalls and abiting member, the biting member configured to be disposed between theuser's mandibular and maxillary teeth, the one or more sidewallsangularly disposed relative to the biting member and configured to becoupled to a plurality of members comprising magnetically permeablematerial; and a second arch-shaped body configured to be worn outside auser's mouth such that the arch-shaped body follows a contour of theuser's face; where the arch-shaped body comprises one or more sidewallsconfigured to be coupled to a plurality of members comprisingmagnetically permeable material. In some embodiments, at least some ofthe plurality of members further comprise ferromagnetic material, suchas, for example, magnets. In some embodiments, at least some of the oneor more sidewalls of either or both the first and second arch-shapedbody generate different magnetic moments.

In some embodiments of the external device, the correction magnets areembedded inside plastic strips which in turn are mounted on the archshaped body outside the mouth. In some embodiments, these strips aredivided into 4 columns for left molars, left incisors, right incisorsand right molars. Each strip can be mounted on the frame individually.

In some of the present hybrid embodiments including intraoral andexternal components (which may be referred to as intraoral-externalhybrid devices or systems), the intraoral component can be configured tomainly correct the magnetic field induced in incisor brackets (e.g., byincluding only or primarily magnets corresponding to the incisors),while the external component can be configured corrects both incisor andmolar brackets (e.g., by including magnets corresponding to both theincisors and the molars).

In some embodiments of the present apparatuses, at least one of the oneor more sidewalls comprises a curved surface, and the apparatus isconfigured to be worn by a user such that normal vectors along thesurface lie substantially in a plane perpendicular to a magnetic fieldof a magnetic resonance imaging scanner.

Some embodiments of the present apparatuses further comprise: aplurality of members coupled to at least one of the one or moresidewalls, the plurality of members comprising magnetically permeablematerial. In some embodiments, at least some of the plurality of memberscomprise ferromagnetic material. In some embodiments, at least some ofthe plurality of members comprise magnets. In some embodiments, themembers comprising magnets are coupled at substantially equal intervalsalong a length of the at least one of the one or more sidewalls. In someembodiments, the members comprising magnets are coupled to the at leastone of the one or more sidewalls in two rows. In some embodiments,between 20 and 28 of the plurality of members comprise magnets. In someembodiments, at least one of the members comprising magnets comprises amaterial with a high intrinsic coercivity. In some embodiments, at leastone of the members comprising magnets comprises neodymium. In someembodiments, at least one of the members comprising magnets is coatedwith nickel or nickel alloy and/or coated with plastic (e.g., paryleneplastic). In some embodiments, at least one of the members comprisingmagnets has a long axis and a magnetization along the long axis, and themagnetization is configured to align in a substantially oppositedirection to a magnetic field of a magnetic resonance imaging scanner.In some embodiments, at least some of the members comprising magnets arecoupled to at least one of the one or more sidewalls such that themembers comprising magnets are in close proximity to brackets of theuser's dental braces when the apparatus is worn by the user. In someembodiments, at least one of the members comprising magnets isconfigured to have a substantially equal but opposite magnetic moment toa bracket of the user's dental braces. Some embodiments further comprisea layer of material configured to be coupled to the at least one of theone or more sidewalls such that the layer of material overlies each ofthe plurality of members. In some embodiments, the plurality of membersis configured to partially restore losses in magnetic field homogeneitycaused by non-biological materials within the user's mouth duringmagnetic resonance imaging. In some embodiments, the plurality ofmembers is configured to reduce artifacts in magnetic resonance imagingimages caused by non-biological materials within the user's mouth duringmagnetic resonance imaging. In some embodiments, the plurality ofmembers is configured to substantially cancel out magnetic fieldsinduced by non-biological materials within the user's mouth duringmagnetic resonance imaging. In some embodiments, a total magnetic momentgenerated by the plurality of members is substantially equal butopposite to the magnetic moment induced by non-biological materialswithin the user's mouth during magnetic resonance imaging. In someembodiments, the non-biological materials within the user's mouthcomprise dental braces.

Some embodiments of the present apparatuses further comprise: a secondarch-shaped body configured to be worn inside of a user's mouth suchthat the second arch-shaped body follows a contour of at least some ofthe user's teeth; where the second arch-shaped body comprises one ormore sidewalls and a biting member, the biting member configured to bedisposed between the user's mandibular and maxillary teeth, the one ormore sidewalls angularly disposed relative to the biting member andconfigured to be coupled to a plurality of members comprisingmagnetically permeable material; and where the second arch-shaped bodydiffers relative to the first arch-shaped body in at least one of: sizeand the configuration in which the plurality of members can be coupledto the one or more sidewalls.

Some embodiments of the present methods comprise: performing magneticresonance imaging on a user having one or more magnets coupled to anapparatus disposed in the user's mouth or outside and adjacent to theuser's mouth, the magnets configured to reduce artifacts in magneticresonance imaging images caused by non-biological materials within theuser's mouth during magnetic resonance imaging. Some embodiments furthercomprise: adjusting the orientation of the user's head by manipulating ahandle coupled to the apparatus.

Some embodiments of the present methods comprise: coupling a pluralityof magnets to an arch-shaped body, the arch-shaped body configured to beworn by a user and the magnets configured to reduce artifacts inmagnetic resonance imaging images caused by non-biological materialswithin the user's mouth during magnetic resonance imaging.

Some embodiments of the present apparatuses comprise a forehead supportconfigured to be worn such that the forehead support follows thecontours of the user's forehead. In some embodiments, the foreheadsupport includes one or more openings to facilitate attachment and/oradjustment of a frame. In some embodiments, the frame further attachesto an arch-shaped body configured to be worn on the outside of a user'sface.

Some embodiments of the present inventions comprise one or more strapsjoined to a forehead support, frame, and/or arch-shaped body in order tosecure a forehead support, frame, and/or arch-shaped body to the outsideof a user's face and/or head. In some embodiments, any or a part of anyof an arch-shaped body, forehead support, frame, or one or more strapsmay be made of non-ferrous material, such as plastic.

Typically the external device fit tightly on patients such that theorthodontic brackets on the maxillary arch may be pressed against theskin in front of them with significant force. A maxillary mouth guardmay be used with the external device to protect the skin. In the hybriddevice described above, the intraoral component serves this function.

The magnetic moments induced in orthodontic appliances are different fordifferent vendors and models of orthodontic appliances. Molar bracketshave a larger range of variability compared with incisor brackets.Embodiments of the present devices (and/or apparatuses and/or systems)can be included in or presented as a kit containing exchangeablecomponents with different magnetic moments. In use for MRI examinations,an appropriate device and/or exchangeable components can be selected andassembled from such a kit based, for example, on a calibration B₀ scanand computer analysis of the scan to best match the braces that thepatient is actually wearing. For example, embodiments of the presentkits can include multiple maxillary and/or mandible pieces for anintraoral device, each of which pieces having different magneticmoments. Similarly, embodiments of the present kits can include multiplestrips or members with embedded magnets of different magnetic momentsfor each mounting position on the frame. By using one particularmaxillary piece and/or mandibular piece, a large range of possible totalmagnetic moments of braces can be matched.

The term “coupled” is defined as connected, although not necessarilydirectly, and not necessarily mechanically; two items that are “coupled”may be unitary with each other. The terms “a” and “an” are defined asone or more unless this disclosure explicitly requires otherwise. Theterm “substantially” is defined as largely but not necessarily whollywhat is specified (and includes what is specified; e.g., substantially90 degrees includes 90 degrees and substantially parallel includesparallel), as understood by a person of ordinary skill in the art. Inany disclosed embodiment, the terms “substantially,” “approximately,”and “about” may be substituted with “within [a percentage] of” what isspecified, where the percentage includes 0.1, 1, 5, 10, and 20 percent.

Further, a device or system that is configured in a certain way isconfigured in at least that way, but it can also be configured in otherways than those specifically described.

The terms “comprise” (and any form of comprise, such as “comprises” and“comprising”), “have” (and any form of have, such as “has” and“having”), “include” (and any form of include, such as “includes” and“including”), and “contain” (and any form of contain, such as “contains”and “containing”) are open-ended linking verbs. As a result, anapparatus that “comprises,” “has,” “includes,” or “contains” one or moreelements possesses those one or more elements, but is not limited topossessing only those elements. Likewise, a method that “comprises,”“has,” “includes,” or “contains” one or more steps possesses those oneor more steps, but is not limited to possessing only those one or moresteps.

Any embodiment of any of the apparatuses, systems, and methods canconsist of or consist essentially of—rather thancomprise/include/contain/have—any of the described steps, elements,and/or features. Thus, in any of the claims, the term “consisting of” or“consisting essentially of” can be substituted for any of the open-endedlinking verbs recited above, in order to change the scope of a givenclaim from what it would otherwise be using the open-ended linking verb.

The feature or features of one embodiment may be applied to otherembodiments, even though not described or illustrated, unless expresslyprohibited by this disclosure or the nature of the embodiments.

Some details associated with the embodiments described above and othersare described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings illustrate by way of example and not limitation.For the sake of brevity and clarity, every feature of a given structureis not always labeled in every figure in which that structure appears.Identical reference numbers do not necessarily indicate an identicalstructure. Rather, the same reference number may be used to indicate asimilar feature or a feature with similar functionality, as maynon-identical reference numbers. The figures are drawn to scale (unlessotherwise noted), meaning the sizes of the depicted elements areaccurate relative to each other for at least the embodiment depicted inthe figures.

FIG. 1 depicts results from three different MRI techniques performed ona patient wearing dental braces.

FIG. 2A is a cutaway perspective view of one embodiment of the presentmagnetic field correction devices.

FIG. 2B is a second cutaway perspective view of the embodiment of FIG.2A shown relative to a patient's teeth with dental braces as worn in apatient's mouth.

FIG. 3 is a cutaway perspective view of a second embodiment of thepresent magnetic field correction devices configured to be worn outsideof and adjacent to a patient's mouth.

FIG. 4 is a cutaway perspective view of a third embodiment of thepresent magnetic field correction devices comprising two sidewalls.

FIG. 5 is a perspective view of a fourth embodiment of the presentmagnetic field correction devices comprising a handle.

FIG. 6 is a graph of the demagnetization curve for a NdFeB (neodymium)magnet.

FIG. 7 depicts sagittal ΔB₀ maps for a simulated patient with dentalbraces and without a magnetic field correction device, with dentalbraces and with a magnetic field correction device, and without dentalbraces and without a magnetic field correction device.

FIG. 8 depicts a magnetic resonance angiography brain slice for apatient, along with a corresponding image for a simulated patientwithout dental braces and without a magnetic field correction device,with dental braces and without a magnetic field correction device, andwith dental braces and with a magnetic field correction device.

FIG. 9A is a perspective view of one embodiment of the present magneticfield correction devices configured to be worn outside of a user's mouthand coupled to a forehead support.

FIG. 9B is a second perspective view of the embodiment of FIG. 9A.

FIG. 10 is a perspective view of a head phantom with dental braces andthe intraoral device.

FIG. 11 is a perspective view the head phantom of FIG. 10 wearing anintraoral-external hybrid embodiment of the present magnetic fieldcorrection devices.

FIG. 12 depicts sagittal ΔB₀ maps for a simulated patient without dentalbraces and without a magnetic field correction device, with dentalbraces and without a magnetic field correction device, and with dentalbraces and with a intraoral-external hybrid magnetic field correctiondevice.

FIGS. 13A-13B depict another embodiment of the present intraoraldevices.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Excessive B₀ inhomogeneity on a macroscopic scale induced bynon-biological materials within a user (a patient undergoing MRI)'smouth, for example, ferromagnetic dental implants and dental braces,results in MRI artifacts [1-7] that can compromise the diagnostic valueof MRI scans [8-9]. This is a special case of the more commonly knownproblem of susceptibility artifacts from surgical implants [10-18]. Themagnitude of resulting loss or distortion depends on the type of MRItechnique [14]. For example, because echo planar imaging (EPI) readoutis sensitive to magnetic susceptibility effects, diffusion tensorimaging and diffusion-weighted images may be most affected. EPI uses lowbandwidth per pixel for readout in the phase encoding direction, andeven small inhomogeneities in the B₀ field can cause noticeabledistortions in MRI images. In gradient echo images, artifacts can beobserved when T2* is decreased to near the echo time (TE), especiallywhen using larger voxel sizes. Typically, the artifacts manifestthemselves as a loss of signal near the non-biological materials (e.g.,the mouth in a patient with dental braces) and displacement ofanatomical structures near the induced signal void (e.g., thehypothalamus area). Further from the signal void, the image distortionsmay be subtle and typically result in poor shimming. In magneticresonance angiography, a frequency selective pulse is used to exciteproton spins in the image volume. The presence of B₀ inhomogeneities cancause the resonance frequency of the proton spins near thenon-biological materials to be shifted outside of the bandwidth of thefrequency selective pulse, causing signal loss.

FIG. 1 depicts image artifacts caused by non-biological materials withina patient's mouth in three different MRI scans: a sagittal T1 weightedimage 10, an axial diffusion-weighted image of the area near the skullbase 14, and a magnetic resonance angiography image 18. The patientundergoing the scans shown in FIG. 1 was wearing dental braces. In thesagittal T1 weighted image 10, the signal from the facial-orbital area22 and the pituitary area 26 is severely distorted and/or missing. Inthe axial diffusion-weighted image 14, significant distortion can beseen near the base 30 of the skull. In the magnetic resonanceangiography image 18, the signal is lost in the area 34 of theintracranial internal carotid arteries.

To correct images losses and distortions (e.g., as shown in FIG. 1),small permanent magnets may be placed near non-biological materials in auser's mouth to cancel out the magnetic fields induced by thenon-biological materials. Factors such as the number of magnets, magnetsize, magnet shape, magnet strength, and/or magnet position (e.g.,orientation) may be selected to significantly reduce and/or eliminatesusceptibility artifacts caused by orthodontic appliances or surgicalimplants in a user's mouth. Small pieces of ferromagnetic materials maybe used in conjunction with the magnets to provide for fine adjustmentsto the magnetic field of the magnetic field devices. For example, FIG.2A depicts a perspective view of one embodiment 38 of the presentmagnetic field correction devices or apparatuses. In the embodimentshown, device 38 comprises an arch-shaped body 42 configured to be worninside of a user's mouth such that the arch-shaped body follows acontour of at least some of the user's teeth 46 (e.g., as shown in FIG.2B). For example, arch-shaped body 42 can resemble a mouth guard whichcan be worn with dental braces 50 in place on the user's teeth. Throughat least the selection of the arch-shaped body 42 size, embodiments ofthe present magnetic field correction devices can be configured to fithuman mouths of various shapes and sizes. For example, older childrengenerally have larger jaw sizes than younger children, and AfricanAmericans tend to have larger jaw sizes than Caucasians and Asians ofsimilar sex and age (who tend to have smaller jaw sizes).

In the embodiment shown, arch-shaped body 42 defines a biting member 54configured to be placed between the user's mandibular and maxillaryteeth (e.g., maxillary teeth 46 a and mandibular teeth 46 b). Bitingmember 54 can be clamped by and between the user's mandibular andmaxillary teeth during use such that arch-shaped 42 body remainssubstantially fixed relative to the user's teeth during an MRI procedure(e.g., clamped between a user's maxillary teeth 46 a and mandibularteeth 46 b as shown in FIG. 2B).

Referring now to FIG. 3, device 38 a is substantially similar to device38, with the primary exception that device 38 a is configured to be wornoutside of a user's mouth such that the arch-shaped body follows acontour of the user's face (e.g., arch-shaped body 42 a is larger thanarch-shaped body 42 to accommodate the lips of a user's mouth, and theback of sidewall 58 a is configured to rest against a user's face nearthe user's mouth). Arch-shaped body 42 a can be fixed to the user's faceby any means which permit the functionality described in thisdisclosure, including, but not limited to, gluing, strapping, and/orthrough the user clamping a biting member similar to biting member 54 indevice 38 (not shown in FIG. 3).

Referring back to FIGS. 2A and 2B, in the embodiment shown, arch-shapedbody 42 comprises one or more sidewalls 58 (e.g., one, as shown) thatare angularly disposed relative to biting member 54 (e.g., as shown inFIG. 2A) and configured to be coupled to a plurality of members 62(e.g., coupled at smooth locations 66, for example, with adhesive)comprising magnetically permeable material (e.g., magnets and/orferromagnetic materials). Other embodiments can be configured to becoupled to a plurality of members 62 in any way which permits thefunctionality described in this disclosure, including, but not limitedto, by pockets, recesses, slots, and/or the like disposed along surface70 of sidewall(s) 58). In the embodiment shown, device 38 includes asingle sidewall 58 that is substantially perpendicular to biting member54. Other embodiments, such as device 38 b shown in FIG. 4, can includetwo or more sidewalls 58 (e.g., front sidewall 58 b and rear sidewall 58c). Other embodiments can have any number of sidewalls which permits thefunctionality described in this disclosure (e.g., upper and lowersidewalls, front and rear sidewalls, and/or the like). Referring back toFIGS. 2A and 2B, in the embodiment shown, sidewall 58 has a thickness of2 millimeters (mm); however, in other embodiments, sidewall(s) 58 canhave any thickness that permits the functionality described in thisdisclosure. In the embodiment shown, the shape of sidewall 58 isconfigured to substantially overlie at least some of maxillary teeth 46a and mandibular teeth 46 b of a user (e.g., covering all of themandibular and maxillary teeth of a user from central incisors 46 c tosecond molars 46 d, as shown in FIG. 2B). In other embodiments, byvarying the height or location of sidewall(s) 58 relative to bitingmember 54, the sidewall(s) can be configured to substantially overlieonly some of maxillary teeth 46 a (e.g., device 38 b) and/or only someof mandibular teeth 46 b of the user. In the embodiment shown, at leastone of sidewall(s) 58 comprises a curved planar surface 70. In thisembodiment, curved planar surface 70 is configured such that normalvectors (e.g., 74) along the surface lie in a plane that issubstantially perpendicular to a magnetic field of a magnetic resonanceimaging scanner when device 38 is worn by a user.

In the embodiment shown, arch-shaped body 42 is molded to fit arepresentative user's mouth (e.g., through molded indentations 54 a inbiting member 54 that correspond to a representative user's teeth 46).For example, arch-shaped 42 body may be molded or otherwise formed froma dental impression (e.g., obtained from a dental model or a patient),which may be representative of the dental structure of a cross-sectionof (e.g., multiple) expected MRI patients. Suitable impression trays canbe obtained from Ortho Technology. After the impression tray has beenmolded from a representative user's teeth, the front surface of theimpression tray can be heated and bent outward to form a sidewall (e.g.,58) with a front surface (e.g., 70) having normal vectors (e.g., 74)that are substantially perpendicular to a magnetic field generatedduring magnetic resonance imaging. In other embodiments, the arch-shapedbody may be formed from a thin sheet of plastic that is vacuum molded tofit a representative user's mandibular teeth 46 b or maxillary teeth 46a (e.g., vacuum molded with a Biostar Vacuum former, available fromGreat Lakes Orthodontics). In these embodiments, the molding process canbe performed twice such that the magnetic field correction devicescomprise two arch-shaped bodies, one for the mandibular teeth of arepresentative user, and one of the maxillary teeth of a representativeuser (e.g., device 38 b, which is molded to fit a representative user'smaxillary teeth). In some embodiments, the present devices can comprisemultiple arch-shaped bodies, sets of arch-shaped bodies (e.g., fortwo-piece devices), and/or devices. In such embodiments, the presentapparatuses can comprise, for example, a larger device or arch-shapedbody corresponding to a representative adult patient and a smallerdevice or arch-shaped body corresponding to a representative childpatient (e.g., each device or arch-shaped body configured to be worn bya different expected user). Multiple devices and/or arch-shaped bodiescan be disposed in a kit, and a physician can select the mostappropriately-sized device for a given patient at the time of use (e.g.,by selecting the device with the arch-shaped body that most closelycorresponds to the given patient's dental structure). In otherembodiments, the arch-shaped body is configured to be molded to fit aparticular user's mouth at the time of use. For example, someembodiments of the present magnetic field correction devices (e.g., 38b) can comprise a thermoplastic material such that the arch-shaped body(e.g., 42 b) can be heated to a deformable plastic state and placedwithin a user's mouth to contour to the user's teeth such that thearch-shaped body is contoured to the user's teeth and returns to asubstantially rigid or inelastic state as it cools (e.g., such that themagnetic field correction device can be worn by a user similarly to asshown in FIG. 2B).

FIG. 5 depicts another embodiment 38 c of the present magnetic fieldcorrection devices. Field correction device 38 c is substantiallysimilar to device 38, with the primary exception that device 38 ccomprises a handle 78 configured to protrude from a user's mouth (e.g.,when the mouth guard is worn by a user as shown in FIG. 2B). Handle 78can be used as a marker for optimally positioning the patient's headduring an MRI which can be desirable for maximizing the effectiveness ofmagnetic field correction as discussed below.

Referring back to FIGS. 2A and 2B, in the embodiment shown, device 38comprises a plurality of members 62 coupled to sidewall 58 where themembers 62 comprise magnetically permeable material. In this embodiment,members 62 are coupled to sidewall 58 with adhesive. One example of asuitable adhesive is Triad Gel, manufactured by Dentsply GAC. In otherembodiments, the members can be coupled to the sidewall(s) by anystructure and/or method that permits the functionality described in thisdisclosure (e.g., through placement of members in receptacles disposedalong surface 70 of the sidewall(s)). In the embodiment shown, at leastsome of the plurality of members comprise ferromagnetic material 62 b(e.g., segments of stainless steel braces arch-wire). Ferromagneticmaterials 62 b may be placed, removed, and/or adjusted to provide fineadjustment of the magnetic field generated by magnetic field correctiondevice 38. For example, magnets 62 a may only be available in a fixedsize and, in some instances, can cause overcorrection of MRI artifacts(e.g., by possessing a fixed size and/or magnetic moment that generatestoo strong of a corrective magnetic field). Thus, ferromagneticmaterials (e.g., 62 b) can enhance the induced magnetic field fromnon-biological materials within the user's mouth to correct anyovercorrections caused by magnets 62 a (e.g., by cancelling any excessmagnetic field generated by magnets 62 a). Additionally, thecharacteristics (e.g., size, shape, mass, material, and/or the like) offerromagnetic materials 62 b can be easily controlled (e.g., byselection, cutting, shaping, and/or the like) to provide fine adjustmentof the magnetic field of the present devices (e.g., by changing themagnetic field induced in the ferromagnetic materials and thus device38). In the embodiment shown, at least some of the plurality of memberscomprise magnets 62 a (e.g., between 20 and 28 members comprisemagnets). In some embodiments, twenty-eight (28) of members 62 comprisemagnets (e.g., one magnet corresponding to each dental bracket in atypical orthodontic treatment consisting of 28 dental brackets). Inother embodiments the present field correction devices may comprise anynumber of members, and any number of the members may compriseferromagnetic materials and/or magnets (e.g., from four (4) to onehundred and twelve (112) members, of which from two (2) to fifty-six(56) comprise magnets). In the embodiment shown, members 62 comprisingmagnets 62 a are coupled at substantially equal intervals 82 along alength (e.g., from left to right along an outer surface) of sidewall(s)58 (e.g., each magnet is spaced a substantially equal distance 82 fromeach adjacent magnet). In the embodiment shown, magnets 62 a are coupledto sidewall 58 such that the magnets form two rows 86 a and 86 b alongsidewall 58. In this embodiment, device 38 also comprises a layer ofmaterial 90 coupled to sidewall 58 (e.g., coupled, as shown, withadhesive), where the layer of material 90 is configured to overlie eachof plurality of members 62. Layer 90 can comprise a plastic sheet (e.g.,a suitable plastic sheet can be obtained from Great Lakes orthodontics).Layer 90 can help ensure that members 62 do not directly contact tissueor saliva, and that members 62 are not swallowed if they come loose fromsidewall 58.

In some of the present embodiments, at least one of the members 62comprising magnets 62 a comprises a material with a high intrinsiccoercivity such that the at least one member can resist demagnetizationin at least a 1.5 T MRI scanner (e.g., a minimum intrinsic coercivity of20 kiloOersted (kOe)). For example, in the embodiment shown, all ofmembers 62 that comprise magnets 62 a comprise a magnetic material withan intrinsic coercivity of at least 20 kOe. In the embodiment shown,magnets 62 a are configured such that the magnetization of each magnetis aligned in a direction that is substantially opposite to the expecteddirection of a B₀ field during an MRI of a patient wearing device 38.Therefore, in this embodiment, device 38 (via magnets 62 a) isconfigured such that magnets 62 a can experience a demagnetizing fieldwhen placed within an MRI scanner, resulting from both an internaldemagnetization field (within magnets 62 a) and an external (B₀)demagnetization field. In some embodiments, such as the one shown, atleast one of members 62 comprising magnets 62 a comprises a NdFeB(neodymium) magnet (e.g., a grade N38EH NdFeB neodymium magnet,available from Dexter Magnetic Technologies). In other embodiments,magnets 62 a can comprise any material which permits the functionalitydescribed in this disclosure. When magnetized, neodymium magnetstypically have a magnetization (magnetic moment per unit volume)comparable to that induced by non-biological materials (e.g., stainlesssteel dental braces brackets) inside an MRI scanner. Additionally,neodymium magnets possess a strong intrinsic coercivity and thus resistirreversible demagnetization in most MRI scanners (e.g., 1.5 T MRIscanners). An irreversibly demagnetized magnet will not return to itsoriginal magnetization when the external demagnetizing magnetic field(e.g., the MRI magnetic field) is removed; however, irreversiblydemagnetized magnets can have their magnetization restored throughapplication of external magnetizing fields.

FIG. 6 is a graph of the demagnetization curve for an N38EH neodymiummagnet [45]. As shown, demagnetization is temperature dependent, and at20° Celsius (° C.) (indicated by the darkened line) the intrinsiccoercivity of an N38EH magnet is 30 kOe, which represents thedemagnetizing field strength at which the magnetization of the N38EHmagnet becomes zero. The “knee point” for a magnet is the demagnetizingfield strength at which the magnet will begin to irreversibly losemagnetization. Below the knee point, the magnet operates in the lineardemagnetization regime, and magnetization can be restored when thedemagnetizing field strength is reduced to zero (e.g., after an MRI scanis complete and the patient is removed from the scanner). The knee pointis not sharply defined, and generally is located in a range from half ofthe intrinsic coercivity to just below the intrinsic coercivity of amaterial. For example, for an N38EH magnet at 20° C., below 26 kOe, theflux density (B) vs. demagnetizing field (H) curve is nearly linear, andthe polarization (J) remains nearly constant. Therefore, 26 kOeapproximates the knee-point for an N38EH magnet at 20° C. At bodytemperature (approximately 37° C.), the intrinsic coercivity of an N38EHmagnet is approximately 27 kOe and the knee point is above 20 kOe,therefore N38EH magnets (e.g., 62 a) will not irreversibly losemagnetization in a 1.5 T MRI scanner (the most common MRI fieldstrength) for a permeance coefficient (p_(c)) greater than 2 (e.g., atthese values, the magnet is operating in the linear regime of thedemagnetization curve) [45, 49]. However, an N38EH magnet canirreversibly lose magnetization at 37° C. (body temperature) in morepowerful MRI scanners (e.g., 3 T or 7 T MRI scanners). In such powerfulMRI scanners, demagnetization of the magnets will occur within seconds;however, the magnets can regain magnetization through placement in anexternal magnetizing magnetic field. Embodiments of the present magneticfield correction devices that have been demagnetized by largedemagnetizing fields (e.g., 3 T or 7 T MRI scanners) may bere-magnetized and can be used again in 1.5 T (or lower B₀) MRI scanners.Magnets with yet higher intrinsic coercivities may be available in thefuture, and may be used with the present field correction devices torestore B₀ homogeneity in 3 T, 7 T, and/or more powerful MRI scanners.

Referring back to FIGS. 2A and 2B, in the embodiment shown, members 62comprising magnets 62 a are coated with nickel or nickel alloy toprolong the shelf-life of the magnets and to reduce the potentialtoxicity of NdFeB magnets [56-57]. For example, such nickel or nickelalloy coated magnets have safely been used in dental devices [48,58-59]. In this embodiment, the nickel or nickel alloy coating isapproximately 0.015 mm thick. In the embodiment shown, at least one ofthe plurality of members 62 comprising magnets 62 a has a long axis 94and a magnetization along the long axis. In the embodiment shown, longaxis 94, and thus the direction of magnetization, of each magnet 62 a isconfigured to align in a substantially opposite direction to thedirection of a magnetic field (B₀) of a magnetic resonance imagingscanner when device 38 is worn by a user. Long and thin magnets (e.g.,62 a) generate a relatively smaller internal demagnetization field(within magnets 62 a) than magnets comprising other shapes. In otherembodiments, the magnets 62 a can comprise any shape that permits thefunctionality described in this disclosure. In some embodiments, magnets62 a can be obtained before they are magnetized, and can thereafter bemagnetized (e.g., by placing the magnets in a 7 T human or 9.4 T animalMRI scanner) before using device 38. Such embodiments can provideconvenience by preventing the magnets from attracting one another whilehandling the magnets and/or assembling the device.

Permanent magnets (e.g., 62 a) experience a torque and a force wheninside an MRI scanner, due to the B₀ field, and the magnitudes of thesephenomena should be limited to ensure patient comfort and safety. Thetorque felt by a patient from a magnetic field correction device isgiven by the following cross-product:

T=m×B ₀  (1)

where m is a vector representing the total magnetic moment of themagnetic field correction device, B₀ is a vector representing the MRImagnetic field, and T is a vector representing the total torque felt bya patient from a magnetic field correction device. Based on Table 1(discussed in more detail below), the total magnetic moment of amagnetic field correction device that corresponds to a dental model with28 Maestro braces brackets is approximately 0.143 Ampere-square meters(A·m²). For example, such a magnetic field correction device maycomprise 28 N38EH NdFeB magnets (e.g., a magnet corresponding to eachbracket).

In a 1.5 T MRI machine, it can be shown that:

T=0.2*sin(Θ)  (2)

where T is torque in Newton-meters (Nm), and Θ is the angle between theB₀ field (B₀) and the magnetic moment (m). As can be seen from Eq. 2,when the magnets (e.g., 62 a) are positioned such that the resultingmagnetic moment from the magnets is oriented 180° from the B₀ field, thetorque experienced by the patient is zero. Human neck muscles useapproximately 5-6 Nm of torque to counter gravitational forces acting onthe head [52]. Additionally, the human biting force, even at 7-12 yearsof age, is at least 350 Newtons (N) [53]. Assuming a leverage distanceof 7 centimeters (cm) (e.g., the approximate width of some embodimentsof the present magnetic field correction devices from one side of thedistal portion of the arch-shaped body 42 to the other), the human jawcan resist a torque of approximately 25 Nm. Therefore, the maximumtorque experienced by a patient wearing an embodiment of the presentmagnetic field correction devices (e.g., 38) can be easily overcome bythe neck and jaw muscles.

The force experienced by a permanent magnet (e.g., 62 a) within an MRImachine is directly proportional to the gradient of the B₀ field.Therefore, the largest force occurs while the patient is moving in andout of the scanner. For a large 3 T MRI magnet, the maximum B₀ gradientis 5.2 teslas per meter (T/m) [55]. In a 1.5 T scanner, the maximum B₀gradient is roughly half of that for a 3 T MRI, or approximately 2.6T/m. Multiplying this value by the total magnetic moment of magneticfield correction device that corresponds to a dental model with 28Maestro braces brackets (e.g., which may comprise 28 NdFeB magnets, asdescribed above) (provided above) results in an approximated maximumforce experienced by a patient wearing such an embodiment of the presentmagnetic field correction devices of only 0.37 N.

Referring back to FIGS. 2A and 2B, in the embodiment shown, theplurality of members 62 is configured to partially restore losses inmagnetic field homogeneity caused by non-biological materials within theuser's mouth (e.g., dental braces 50) during magnetic resonance imaging.FIG. 7 depicts sagittal ΔB₀ maps, which indicate magnetic field (B₀)homogeneity, including a map 98 for a simulated patient with dentalbraces and without a magnetic field correction device, a map 102 for asimulated patient with dental braces and with a magnetic fieldcorrection device, and a map 106 for a simulated patient without dentalbraces and without a magnetic field correction device. To simulate apatient with dental braces, a realistic dental brace model from DamonSystem can be used, which has twenty-eight (28) stainless steel dentalbraces brackets mounted on twenty-eight (28) model teeth. The headphantom (used by the American College of Radiology for accreditation[47], an example of which is shown in FIGS. 7 and 8) can be positionedin an eight channel SENSE head coil, and the dental model can bepositioned next to the phantom to simulate a patient in a supineposition. More recent work used a 3D printed head phantom (FIG. 10). TheΔB₀ sagittal maps can be measured from the phase difference of twoimages with different echo times (e.g., 4.6 milliseconds (ms) and 5.0ms) acquired with a fast field echo (FFE) sequence, and low signal areasin the amplitude images can be excluded. The sagittal map 98 for thesimulated patient with dental braces shows large regions 110 of imageloss, as well as image distortion across the map (e.g., compare map 98with map 106). For map 98, the peak-to-peak ΔB₀ is 81.0parts-per-million (ppm), and the standard deviation is 9.1 ppm. Therelatively large peak-to-peak ΔB₀ value is indicative of a loss ofhomogeneity in the MRI magnetic field due to the magnetic fields inducedby the dental braces. Sagittal ΔB₀ map 102 is of a simulated patientwith dental braces and with one of the present magnetic field correctiondevices (e.g., 38) disposed within the simulated patient's mouth. Map102 shows a reduced region 114 of image loss, as well as significantlyreduced distortion throughout the image (e.g., compare map 102 with map98). For map 102, the peak-to-peak ΔB₀ value is 20.3 ppm, and thestandard deviation is 1.4 ppm. The significant reduction in peak-to-peakΔB₀ from map 98 to map 102 is indicative of the magnetic fieldcorrection device partially restoring homogeneity to the MRI magneticfield. Sagittal map 106 for the simulated patient without braces andwithout a magnetic field correction device has a peak-to-peak ΔB₀ valueof 6.6 ppm and a standard deviation of 1.2 ppm. As shown in FIG. 7, themagnetic field correction devices can be configured to significantlyimprove B₀ homogeneity (e.g., by varying at least the number, size,shape, strength, and/or positioning (e.g., orientation) of magnets 62 aand/or ferromagnetic materials 62 b).

Referring back to FIGS. 2A and 2B, the plurality of members 62 isfurther configured to reduce artifacts in magnetic resonance imagingcaused by non-biological materials within the user's mouth (e.g., dentalbraces 50) during magnetic resonance imaging. FIG. 8 depicts a magneticresonance angiography brain slice 118 for a patient, along with acorresponding image 122 for a simulated patient without dental bracesand without a magnetic field correction device, image 126 for asimulated patient with dental braces and without a magnetic fieldcorrection device, and image 130 for a simulated patient with dentalbraces and with a magnetic field correction device. Axial brain slice118 is through the middle of the pons, where magnetic resonanceangiography is particularly important from a clinical perspective. Asshown image 122 for a simulated patient without braces and without amagnetic field correction device is clear, and there is no significantimage loss or distortion. In image 126 for a simulated patient withdental braces and without a magnetic field correction device, there is alarge region 134 of image loss, as well as distortions throughout theimage. In magnetic resonance angiography (as used in FIG. 8), afrequency selective radio frequency (RF) pulse is used for signalexcitation or suppression. During these imaging techniques,non-biological materials (such as dental braces) can result in artifacts(e.g., region 134) by shifting the water resonance position to outsideof the excitation bandwidth. With an embodiment of the present magneticfield correction devices (e.g., 38) disposed in the simulated patient'smouth along with dental braces, image 130 shows a large reduction inboth image loss in the vicinity of the dental braces 138, as well as areduction in image distortion throughout the image (e.g., compare image130 with image 126). As shown, the present magnetic field correctiondevices can be configured to reduce artifacts (e.g., region 134) inmagnetic resonance imaging caused by non-biological materials within theuser's mouth (e.g., dental braces 50) during magnetic resonance imaging(e.g., by varying at least the number, size, shape, strength, and/orpositioning (e.g., orientation) of magnets 62 a and/or ferromagneticmaterials 62 b)

Referring back to FIGS. 2A and 2B, in the embodiment shown, thenon-biological materials within the user's mouth comprise dental braces(e.g., dental braces 50). An example of such dental braces aremanufactured by Ormco. In the embodiment shown, members 62 comprisingmagnets 62 a are coupled to sidewall 58 such that, if device 38 is worn(e.g., as shown in FIG. 2B) in the mouth of a user with dental braces,each of magnets 62 a is in close proximity to a different one ofbrackets 50 a of the user's dental braces 50 (e.g., magnet 146 is inclose proximity to bracket 142 in FIG. 2B). As used in this disclosure,“close proximity” means between 0 mm and 15 mm between the magnet andthe bracket. Both guide-wire 50 b and brackets 50 a of dental braces 50can contribute to an induced magnetic field generated during MRI, butthe contributions from the guide-wire are generally small. Additionally,it is relatively easy to remove the guide-wire from the patient's bracesbefore performing an MRI scan. Therefore, the induced magnetic field maybe dominated by the contributions from the brackets. Thus, the totalinduced magnetic field by the dental braces can be approximated as thesum of the magnetic dipole field from each of brackets 50 a. In an idealcase, a magnet would be positioned precisely in the same location aseach dental bracket with an equal but opposing magnetic moment to thebracket to completely cancel out the magnetic moment induced by eachindividual bracket; in practice, however, this is difficult if notimpossible. Therefore, placing the magnets as close as practicable tobrackets can be desirable.

In the embodiment shown, each member 62 comprising a magnet 62 a isconfigured to have a substantially equal but opposite magnetic moment toa bracket (e.g., 142) on the user's dental braces 50. Examples ofcommonly used brackets for dental braces are available from DentsplyGAC, 3M Unitek, and American Orthodonics. Each tooth in a patient'smouth has a unique anatomical shape, and therefore brackets in a set ofdental braces can comprise different designs from one another (e.g., toprovide optimal bonding to each individual tooth). Additionally,brackets made in different manufacturing batches may not be identicaland can possess variations in shape, weight, and therefore inducedmagnetic moment within an MRI scanner. The magnetic properties of adental braces bracket can also vary with bracket orientation. Sincevariability of dental braces bracket orientation may be inevitable,given neither a tooth, nor the dental braces bracket mounted on thetooth will be perfectly straight, it can be desirable to determine theinduced field pattern for a given dental braces bracket in multipleorientations (not just one orientation that corresponds to straightlymounting the bracket on a straight tooth).

The magnetic properties of a given dental braces bracket including theeffect of orientation and manufacturing-related variability on theinduced magnetic dipole, as well as induced magnetic dipole amplitudecan be determined. MRI machines are sensitive to the z-component ofinduced magnetic fields, and dental braces bracket magneticsusceptibility can be anisotropic: dental braces brackets can generatedifferent non-zero magnetic moments in the x and y directions, and eachcan generate a magnetic field with a z-component. To measure themagnetic properties of a dental braces bracket, a 2 liter (l) sphericalglass flask containing a water solution of NaCl doped with ProHance canbe used. A dental bracket can be mounted, in the desired orientation, onthe tip of a plastic rod, which can be inserted into the center of theflask through a thin nuclear magnetic resonance (NMR) spectroscopy tube.The B₀ field can then be mapped using a three-dimensional (3D) gradientecho sequence at two echo times of 3.5 ms and 3.8 ms respectively,utilizing the following parameters: 3D coronal FFE, cubic voxel size of8 mm², field of view of 224 pixels by 224 pixels, 75 slices, water-fatshift of 0.26 pixels, repetition time (TR) of 10 ms, flip angle of 10°,readout along the right-to-left (RL) direction, number of signalaverages (NSA) of 1, and an acquisition time of 2 minutes (min) and 50seconds (s). The 3D field map can then be obtained from the differenceof phase images and modeled as a magnetic dipole with the equation:

$\begin{matrix}{B_{z} = {\frac{\mu_{0}}{4\pi}\frac{{3{n_{z}\left( {n \cdot m} \right)}} - m_{z}}{|x|^{3}}}} & (3)\end{matrix}$

where x is a displacement vector from the location of the dipole to apoint in space where the field is measured, n is a unit vector along thedirection of x, and m is a vector representing the induced magneticmoment to be determined (e.g., through a least squares fitting routine).Through this process, the magnetic properties of each dental bracesbracket can be measured individually in multiple orientations. The abovemethod can also be used to determine the magnetic properties of amagnet.

Table 1 provides the induced magnetic moment of a Maestro UL1 dentalbracket for five orientations in a 1.5 T MRI scanner.

TABLE 1 Orientation Dependence of the Magnetic Moment of a Maestro UL1Dental Bracket in a 1.5T MRI Scanner −30° Pitch 30° Pitch Straight AngleAngle −30° Roll Angle 30° Roll Angle m_(x) m_(y) m_(z) m_(x) m_(y) m_(z)m_(x) m_(y) m_(z) m_(x) m_(y) m_(z) m_(x) m_(y) m_(z) −0.05 −0.05 5.11−0.04 −0.33 5.11 −0.03 0.29 5.16 −0.03 −0.04 4.96 −0.02 −0.02 5.14Units of magnetic moment are in 10⁻³ A·m², and the bracket orientationangles are defined from the patient's perspective and correspond to apatient lying supine and head first into the MRI magnet. The x-axispoints to left, the y-axis is along the anterior direction, and thez-axis points to the patient's foot. Table 1 shows that m_(x) and m_(y)are much smaller than m_(z), and m_(z) is not sensitive to orientation,for example, the coefficient of variation of m_(z) is about 1.5% forthis Maestro UL1 dental bracket. Therefore, in embodiments in which atleast one of the plurality of members 62 comprising a magnet 62 a isconfigured to have a substantially equal but opposite magnetic moment toa bracket (e.g., a Maestro UL1 dental bracket, as described in Table 1,as bracket 142) on the user's dental braces 50 (e.g., magnetic fieldcorrection device 38), the at least one member is a magnet with amagnetic moment (m_(z)) of approximately −5.11 to −5.14 10⁻³ A·m².

In the embodiment shown, members 62 are further configured tosubstantially cancel out magnetic fields induced by non-biologicalmaterials within the user's mouth during magnetic resonance imaging(e.g., by configuring members 62 comprising magnets 62 a such thatmagnets 62 a have a substantially equal, but opposite, magnetic momentto the magnetic moment induced by the respective dental braces bracketsas described above).

In the embodiment shown, the total magnetic moment generated by members62 is substantially equal but opposite to the magnetic moment induced bynon-biological materials within the user's mouth during magneticresonance imaging. For example, if the non-biological materials comprisedental braces (e.g., 50), it may not be practical to include a magnetfor each corresponding bracket (e.g., 142) on the dental braces.Therefore, the total magnetic moment of the magnetic field correctiondevice can be configured to match the magnetic moment induced by thenon-biological materials (e.g., braces 50) within the user's mouth,regardless of the number of members 62 (e.g., by varying at least thenumber, size, shape, strength, and/or positioning (e.g., orientation) ofmagnets 62 a and/or ferromagnetic materials 62 b). For example, usingthe data from Table 1 and roughly approximating each bracket as aMaestro UL1 bracket, the total magnetic moment induced by 28 brackets isapproximately 0.143 A·m². Therefore, in embodiments of the presentmagnetic field correction devices where the plurality of members 62 areconfigured to generate a substantially equal but opposite magneticmoment to that induced by non-biological materials within the user'smouth during an MRI (e.g., braces 50 consisting of Maestro UL1brackets), the plurality of members can have a total magnetic moment ofapproximately −0.143 A·m².

From data depicted in Table 1, computer simulations can be performed toquantify B₀ inhomogeneity and the dependence of magnetic fieldcorrection effectiveness on head orientation, magnet strength, and/ormagnet location. To perform a computer simulation, an existing 3D T1weighted magnetic resonance image set of the brain can be chosen (e.g.,of a typical 14 year old boy). Twenty-eight magnetic dipoles can beplaced in the position of teeth on the MRI to represent the brackets ondental braces, and each dipole can be assumed to have the m_(z) valuefrom Table 1. The brain can then be segmented into compartments, and foreach region the range, mean, and standard deviation of the inducedmagnetic field can be calculated.

TABLE 2 Magnetic Field Inhomogeneity (ppm) due to Dental Braces andMagnetic Field Correction Effectiveness for Various Head and MagnetLocations, and Magnet Strengths Frontal Temporal Pituitary Central LobeLobe Brainstem glands Brain Cerebellum Braces B₀ range  0.9, 44.1 −3.8,30.4 −11.2, 5.4    7.9, 13.8  1.1, 17.5 −9.9, 0.2 without mean ± sd  8.9± 6.3  2.3 ± 4.5 −1.4 ± 3.2 10.9 ± 1.5  5.3 ± 3.3 −3.3 ± 1.5 correctionShift B₀ range −1.3, 1.9 −0.7, 4.1  −0.3, 0.2  −0.3, 0.0  0.0, 0.8 −0.3,0.0 magnets mean ± sd  0.2 ± 0.3  0.3 ± 0.5 −0.1 ± 0.1 −0.1 ± 0.1  0.2 ±0.1 −0.1 ± 0.1 5 mm outward Pitch head B₀ range  −4.4, −0.3 −7.7, −0.3−2.9, −0.4 −4.3, −3.3 −3.0, −0.6  −2.2, −0.0 by 10° mean ± sd −1.0 ± 0.6−1.6 ± 1.2 −1.7 ± 0.4 −3.8 ± 0.2 −1.4 ± 0.4 −0.5 ± 0.3 10% B₀ range−2.2, 1.8 −6.6, 0.6  −3.5, −1.1 −3.7, −2.7 −2.0, −0.1  −2.6, −0.4 weakermean ± sd −0.0 ± 0.5 −1.3 ± 0.8 −2.1 ± 0.4 −3.1 ± 0.2 −0.8 ± 0.3 −1.1 ±0.4 magnets 10% B₀ range  −7.7, −0.5 −9.0, −0.1 −2.9, 0.4  −5.4, −3.8−3.9, −0.5 −2.1, 0.4 stronger mean ± sd −1.6 ± 1.0 −1.3 ± 1.1 −1.4 ± 0.6−4.6 ± 0.3 −1.6 ± 0.6  0.3 ± 0.3 magnets

The central brain in Table 2 consists of the corpus callosum, basalganglia, and thalami. As shown in Table 2, even in less than idealconditions (e.g., magnets 62 a placed relatively far away from dentalbraces brackets 50 a), B₀ inhomogeneity can be significantly decreasedby the present magnetic field correction devices, and accurate headorientation and magnet (e.g., 62 a) magnetization can be desirable tohelp achieve magnetic field correction effectiveness.

Some embodiments of the present methods comprise performing an MRI on auser (e.g., with an MRI scanner such as a 1.5 T Phillips Achieva) havingone or more magnets (e.g., 62 a) coupled to an apparatus (e.g., magneticfield correction device 38, 38 a, 38 b, or 38 c) disposed in the user'smouth (e.g., as shown in FIG. 2B) or outside and adjacent to the user'smouth (e.g., device 38 a, when worn by a user), where the magnets areconfigured to reduce artifacts in magnetic resonance imaging imagescaused by non-biological materials within the user's mouth duringmagnetic resonance imaging (e.g., in a similar fashion as describedabove). Further embodiments comprise adjusting the orientation of theuser's head by manipulating a handle (e.g., 78) coupled to the apparatus(e.g., magnetic field correction device 38 a) and configured to protrudefrom the user's mouth when the apparatus is worn by the user (e.g.,worn, as shown, in FIG. 2B).

Other embodiments of the present methods comprise coupling a pluralityof magnets (e.g., 62 a) to an arch-shaped body (e.g., 42) configured tobe worn by a user (e.g., worn, as shown, in FIG. 2B) and the magnets(e.g., 62 a) configured to reduce artifacts in magnetic resonanceimaging images caused by non-biological materials (e.g., dental braces50) within the user's mouth during magnetic resonance imaging (e.g., ina similar fashion as described above). In further embodiments, themagnets are selected and placed on the arch-shaped body (e.g., 42) afterdetermining the magnetic moment induced by non-biological materialswithin a user's mouth through use of an MRI measurement (as describedabove or through an iterative process in which an MRI is performed onthe patient and magnet 62 a number, size, shape, strength, and/orpositioning (e.g., orientation) are adjusted until the MRI scans achievea quality necessary for diagnostic value). In other embodiments, anapparatus comprising multiple arch-shaped bodies, sets of arch-shapedbodies, and/or devices can be provided. In such embodiments, thearch-shaped bodies and/or devices can have different magneticcharacteristics (e.g., different magnetic moments), differentconfigurations (e.g., a different configuration in which magneticallypermeable members 62 can be coupled to the arch-shaped bodies, such as,for example, differing smooth locations 66) and/or different physicalcharacteristics (e.g., size, shape, contour, curvature, and/or thelike). The magnetic moment induced by non-biological materials within apatient's mouth can be determined through use of an MRI measurement,and/or the jaw size and/or other relevant physical traits of thepatient's mouth and/or the non-biological materials within the patient'smouth can be assessed. The arch-shaped body, set of arch-shaped bodies,or device that best corresponds to the patient can be selected to beworn by the patient during the MRI.

Patients undergoing an MRI can be from various ethnic backgrounds, andcan have non-biological materials within their mouths (e.g., Ormcodental braces). These patients should not have contra-indications forreceiving an MRI, such as a weak jaw or neck, certain metal implants inthe craniofacial or neck regions, or female subjects who are pregnant orpossibly pregnant. Patients who would require sedation before the MRIscan or who are unconscious may be fitted with an embodiment of thepresent external magnetic field correction devices or apparatuses likethe one shown in FIGS. 9A and 9B.

FIG. 9A depicts a perspective view of an embodiment of the presentmagnetic field correction devices and apparatuses 200 configured to beworn outside a user's mouth (e.g. an external embodiment). Device 200comprises an arch-shaped body 201, which is contoured to fit over auser's face and may be located substantially over a user's mandible andmaxilla, partially or substantially above a user's maxilla, or partiallyor substantially below a user's mandible. The depicted embodiment isconfigured such that, when located partially or substantially above orover a user's maxilla (or above or over the user's teeth), thearch-shaped body does not block or interfere with airflow through theuser's mouth. Similarly, the present embodiments can be configured suchthat, when located partially or substantially below a user's mandible,the arch-shaped body does not block or interfere with airflow throughthe user's mouth or nose.

In the embodiment shown, arch-shaped body 201 is coupled to foreheadsupport 202 via substantially rigid frame 203. Forehead support 202 iscontoured to fit over the user's forehead and includes a plurality ofopenings 204 for coupling forehead support to frame 203 via a pluralityof screw-and nut fasteners 205. The plurality of openings 204 permitframe 203 to couple with forehead support 202 at various locations tofacilitate adjustment of frame 203 relative to the user's head. In thisembodiment, screw-and-nut fasteners 205 are also employed to couple aplurality of members 207 comprising magnetically permeable material(shown in FIG. 9B) to arch-shaped body 201. Screw-and-nut fasteners 205may be secured and removed by hand. In the depicted embodiment, device200 further comprises straps 206, which are coupled to the ends ofarch-shaped body 201 and forehead support 202, and are configured tosecure arch-shaped body 201 and forehead support 202 to the user's headby wrapping around the back of the user's head and fastening tothemselves by, for example, loop-and-fastener means. FIG. 9B depicts aperspective view of device 200 facing away from the user's face. Asshown in FIG. 9B, arch-shaped body 201 is coupled to a plurality ofmembers 207 comprising magnetically permeable material (e.g., permanentmagnets). In some of the present external embodiments, such as the oneshown in FIGS. 9A-9B, members 207 include strips (e.g., plastic strips)in which magnets are disposed or sealed, and the members 207 are in turnmounted on the arch shaped body outside the mouth. In the embodimentshown, these members (207) include four (4) members 207, one each forthe user's left molars, left incisors, right incisors and right molars.In this embodiment, each strip can be mounted on the frame individually.

Device 200 or another external embodiment may also be used inconjunction or combination with an intraoral embodiment of the presentdevices and apparatuses to form a hybrid embodiment. Relative to anintraoral embodiment like that shown in FIG. 3A, an external embodimentof the present devices and apparatuses, like that shown in FIG. 9,permits greater freedom in the placement of magnets and otherferromagnetic materials to better compensate for variability of magneticmoments of orthodontic braces or other non-biological materials in auser's mouth. However, relative to external embodiments of the presentdevices and apparatuses, intraoral embodiments allow closer proximitybetween non-biological materials in a user's mouth and the magnets orother ferromagnetic materials of the devices or apparatuses. When usedin combination, the advantages of both intraoral and externalembodiments of the present devices and apparatuses can be utilized.

As shown in FIGS. 9A and 9B, in the depicted embodiment of device 209,arch-shaped body 210 includes a flexible (e.g., soft or otherwisepliable) link or joint 209 (e.g., at or near the lateral center of thebody, as shown). In such embodiments, link or joint 209 is more flexiblethan other parts of arch-shaped body 201 (which may, for example,include rigid plastic) to allow body 201 (via link or joint 209) to flexto accommodate different maxillary (or mandibular) arch widths inpatients.

FIG. 11 depicts a hybrid external and intraoral embodiment of thepresent devices and apparatuses affixed to the head phantom shown inFIG. 10. As described previously, a head phantom such as that shown inFIGS. 10 and 11 in this application, simulates a patient withtwenty-eight (28) stainless steel dental braces brackets mounted ontwenty-eight (28) model teeth. The brain phantom was fitted with threedifferent sized dental models (one of average size, one 8.5% bigger, andone 8.5% smaller) to investigate whether a one-size hybrid embodimentwould sufficiently counter B₀ distortions in MRI images. The brainphantom shown in FIGS. 10 and 11 was filled with agar gel and evaluatedfor B₀ homogeneity in a similar manner shown and described above withreference to FIG. 7. In some such hybrid embodiments including intraoraland external components (which may be referred to as intraoral-externalhybrid devices or systems), the intraoral component can be configured tomainly correct the magnetic field induced in incisor brackets (e.g., byincluding only or primarily magnets corresponding to the incisors),while the external component can be configured corrects both incisor andmolar brackets (e.g., by including magnets corresponding to both theincisors and the molars).

The magnetic moments induced in orthodontic appliances are different fordifferent vendors and models of orthodontic appliances. Molar bracketshave a larger range of variability compared with incisor brackets.Embodiments of the present devices (and/or apparatuses and/or systems)can be included in or presented as a kit containing exchangeablecomponents with different magnetic moments. In use for MRI examinations,an appropriate device and/or exchangeable components can be selected andassembled from such a kit based, for example, on a calibration B₀ scanand computer analysis of the scan to best match the braces that thepatient is actually wearing. For example, embodiments of the presentkits can include multiple maxillary and/or mandible pieces for anintraoral device, each of which pieces having different magneticmoments. Similarly, embodiments of the present kits can include multiplestrips or members with embedded magnets of different magnetic momentsfor each mounting position on the frame. By using one particularmaxillary piece and/or mandibular piece, a large range of possible totalmagnetic moments of braces can be matched.

FIG. 12 shows the resulting sagittal ΔB₀ maps. In particular, map 221depicts the baseline results, i.e., when the brain phantom was notfitted with braces or an embodiment of the magnetic field correctiondevice. Map 221 has a peak-to-peak ΔB₀ value of 13.8 parts per million(ppm) and standard deviation of 0.40 ppm. Map 222 depicts B₀ homogeneityfor the brain phantom when fitted with braces and without attaching anyembodiment of the magnetic field correction device. Map 222 has apeak-to-peak ΔB₀ value of between 28.7 and 32.2 parts per million (ppm),and standard deviation of between 3.0 and 3.1 ppm for the three dentalmodel sizes. The relatively large peak-to-peak ΔB₀ value (especiallycompared to that of map 221) is indicative of a loss of homogeneity inthe MRI magnetic field due to the magnetic fields induced by the dentalbraces. Map 223 depicts B₀ homogeneity for the brain phantom when fittedwith both braces and a hybrid embodiment of the magnetic fieldcorrection device. Map 223 shows reduced regions 225 of image distortion(e.g., compare map 223 with map 222). For map 223, the peak-to-peak ΔB₀value is 12.7 to 17.3 parts per million (ppm), with a standard deviationof 0.64 to 0.76 ppm. The significant reduction in peak-to-peak ΔB₀ andfrom map 222 to map 223 is indicative of the magnetic field correctiondevice partially restoring homogeneity to the MRI magnetic field; and isan even greater restoration than that shown in FIG. 7 when using only anintraoral magnetic field correction device. As shown in FIG. 12, ahybrid magnetic field correction devices can be configured tosignificantly improve B₀ homogeneity (e.g., by varying at least thenumber, strength, and/or positioning of magnets 62 a and/orferromagnetic materials 62 b in both the intraoral and externalfeatures). The single-sized hybrid device was sufficient to adequatelycorrect B₀ distortions.

In another related study, image quality with and without the braces,undergoing MRI scans for headaches/seizures in two of the patients, anoptic nerve tumor in one of the patients, and a thalamic tumor in thelast patient. The patients tolerated well the use of an externalmagnetic correction device and its use improved the quality of variousMRI images, including orbital, sub-frontal, and anterior temporal MRIs,and, in particular, DWI.

FIGS. 13A-13B depict another embodiment 38 d of the present intraoraldevices. In this embodiment, arch-shaped body 42 d includes two gaps orslits 300 to create flexible (e.g., soft or otherwise pliable) links orjoints (e.g., at or near the lateral center of the body, as shown). Insuch embodiments, these gaps are more flexible than other parts ofarch-shaped body 42 d (which may, for example, include rigid plastic) toallow body 42 d (via the gaps 300) to flex to accommodate differentmaxillary (or mandibular) arch widths in patients.

In some embodiment, the intraoral device is attached to a string that isconfigured to be tethered down around the neck of the patient as asafety measure. In case the device comes out of the patient's mouth, thestring will prevent the device from moving away by the attraction forcebetween the MRI magnet and the device. In some embodiments, theintraoral device is strengthened with extra coating using rubber orother materials to prevent to magnets from escaping or being released ifthe device breaks. The device may also be contained inside fabric pouchduring use for the same purpose.

The above specification and examples provide a complete description ofthe structure and use of illustrative embodiments. Although certainembodiments have been described above with a certain degree ofparticularity, or with reference to one or more individual embodiments,those skilled in the art could make numerous alterations to thedisclosed embodiments without departing from the scope of thisinvention. As such, the various illustrative embodiments of the methodsand systems are not intended to be limited to the particular formsdisclosed. Rather, they include all modifications and alternativesfalling within the scope of the claims, and embodiments other than theone shown may include some or all of the features of the depictedembodiment. For example, elements may be omitted or combined as aunitary structure, and/or connections may be substituted. Further, whereappropriate, aspects of any of the examples described above may becombined with aspects of any of the other examples described to formfurther examples having comparable or different properties and/orfunctions, and addressing the same or different problems. Similarly, itwill be understood that the benefits and advantages described above mayrelate to one embodiment or may relate to several embodiments.

The claims are not intended to include, and should not be interpreted toinclude, means-plus- or step-plus-function limitations, unless such alimitation is explicitly recited in a given claim using the phrase(s)“means for” or “step for,” respectively.

REFERENCES

These references, to the extent that they provide exemplary proceduralor other details supplementary to those set forth herein, arespecifically incorporated herein by reference.

-   1. New P F, Rosen B R, Brady T J, Buonanno F S, Kistler J P, Burt C    T, Hinshaw W S, Newhouse J H, Pohost G M, and Taveras J M: Potential    hazards and artifacts of ferromagnetic and nonferromagnetic surgical    and dental materials and devices in nuclear magnetic resonance    imaging. Radiology 1983; 147(1): p. 139-48. PMID: 6828719-   2. Fache J S, Price C, Hawbolt E B, and Li D K: MR imaging artifacts    produced by dental materials. AJNR Am J Neuroradiol 1987; 8(5): p.    837-40. PMID: 3118677.-   3. Hinshaw D B, Jr., Holshouser B A, Engstrom H I, Tjan A H,    Christiansen E L, and Catelli W F: Dental material artifacts on MR    images. Radiology 1988; 166(3): p. 777-9. PMID: 3340777.-   4. Shellock F G: Prosthetic heart valves and annuloplasty rings:    assessment of magnetic field interactions, heating, and artifacts at    1.5 Tesla. J Cardiovasc Magn Reson 2001; 3(4): p. 317-24. PMID:    11777223.-   5. Shellock F G: Metallic neurosurgical implants: evaluation of    magnetic field interactions, heating, and artifacts at 1.5-Tesla. J    Magn Reson Imaging 2001; 14(3): p. 295-9. PMID: 11536406.-   6. Hashemi R H, Bradley W G, and Lisanti C J: MRI: The Basics. 3rd    ed. 2010: Lippincott Williams & Wilkins.-   7. Klinke T, Daboul A, Maron J, Gredes T, Puls R, Jaghsi A, and    Biffar R: Artifacts in magnetic resonance imaging and computed    tomography caused by dental materials. PLoS One 2012; 7(2): p.    e31766. PMCID: PMC3285178.-   8. Blankenstein F H, Truong B, Thomas A, Schroder R J, and Naumann    M: Signal loss in magnetic resonance imaging caused by intraoral    anchored dental magnetic materials. Rofo 2006; 178(8): p. 787-93.    PMID: 16862505.-   9. Bateman L M, Latchaw R, and Seyal M: Dental hardware complicating    diagnosis in refractory gelastic epilepsy secondary to hypothalamic    hamartoma. Clin EEG Neurosci 2010; 41(3): p. 151-4. PMID: 20722350.-   10. Laakman R W, Kaufman B, Han J S, Nelson A D, Clampitt M, O'Block    A M, Haaga J R, and Alfidi R J: MR imaging in patients with metallic    implants. Radiology 1985; 157(3): p. 711-4. PMID: 4059558.-   11. Shellock F G: MR imaging of metallic implants and materials: a    compilation of the literature. AJR Am J Roentgenol 1988; 151(4): p.    811-4. PMID: 3048071.-   12. Fellner C, Behr M, Fellner F, Held P, Handel G, and Feuerbach S:    Artifacts in MR imaging of the temporomandibular joint caused by    dental alloys: a phantom model study at T1.5. Rofo 1997; 166(5): p.    421-8. PMID: 9198515.-   13. Saito M, Ono S, Kayanuma H, Honnami M, Muto M, and Une Y:    Evaluation of the susceptibility artifacts and tissue injury caused    by implanted microchips in dogs on 1.5 T magnetic resonance imaging.    J Vet Med Sci 2010; 72(5): p. 575-81. PMID: 20086326.-   14. Hargreaves B A, Worters P W, Pauly K B, Pauly J M, Koch K M, and    Gold G E: Metal-induced artifacts in MRI. AJR Am J Roentgenol 2011;    197(3): p. 547-55. PMID: 21862795.-   15. Buckwalter K A, Lin C, and Ford J M: Managing postoperative    artifacts on computed tomography and magnetic resonance imaging.    Semin Musculoskelet Radiol 2011; 15(4): p. 309-19. PMID: 21928156.-   16. Hecht S, Adams W H, Narak J, and Thomas W B: Magnetic resonance    imaging susceptibility artifacts due to metallic foreign bodies. Vet    Radiol Ultrasound 2011; 52(4): p. 409-14. PMID: 21382122.-   17. Bagheri M H, Hosseini M M, Emami M J, and Foroughi A A: Metallic    artifact in MRI after removal of orthopedic implants. Eur J Radiol    2012; 81(3): p. 584-90. PMID: 21146947.-   18. David F H, Grierson J, and Lamb C R: Effects of surgical    implants on high-field magnetic resonance images of the normal    canine stifle. Vet Radiol Ultrasound 2012; 53(3): p. 280-8. PMID:    22372640.-   19. Mirvis S E, Geisler F, Joslyn J N, and Zrebeet H: Use of    titanium wire in cervical spine fixation as a means to reduce MR    artifacts. AJNR Am J Neuroradiol 1988; 9(6): p. 1229-31. PMID:    3143247.-   20. Wichmann W, Von Ammon K, Fink U, Weik T, and Yasargil G M:    Aneurysm clips made of titanium: magnetic characteristics and    artifacts in MR. AJNR Am J Neuroradiol 1997; 18(5): p. 939-44. PMID:    9159374.-   21. Rudisch A, Kremser C, Peer S, Kathrein A, Judmaier W, and    Daniaux H: Metallic artifacts in magnetic resonance imaging of    patients with spinal fusion. A comparison of implant materials and    imaging sequences. Spine (Phila Pa. 1976) 1998; 23(6): p. 692-9.    PMID: 9549791.-   22. Immel E and Melzer A: Improvement of the MR imaging behavior of    vascular implants. Minim Invasive Ther Allied Technol 2006;    15(2): p. 85-92. PMID: 16754191.-   23. Ernstberger T, Buchhorn G, Baums M H, and Heidrich G: In-vitro    MRI detectability of interbody test spacers made of carbon    fibre-reinforced polymers, titanium and titanium-coated carbon    fibre-reinforced polymers. Acta Orthop Belg 2007; 73(2): p. 244-9.    PMID: 17515239.-   24. Ernstberger T and Heidrich G: Postfusion magnetic resonance    imaging artifacts caused by a titanium, cobalt-chromium-molybdenum,    and carbon intervertebral disc spacer. J Spinal Disord Tech 2007;    20(2): p. 154-9. PMID: 17414986.-   25. Ernstberger T, Heidrich G, Bruening T, Krefft S, Buchhorn G, and    Klinger H M: The interobserver-validated relevance of intervertebral    spacer materials in MRI artifacting. Eur Spine J 2007; 16(2): p.    179-85. PMCID: PMC2200688.-   26. Ernstberger T, Heidrich G, and Buchhorn G: Postimplantation MRI    with cylindric and cubic intervertebral test implants: evaluation of    implant shape, material, and volume in MRI artifacting—an in vitro    study. Spine J 2007; 7(3): p. 353-9. PMID: 17482121.-   27. Ernstberger T, Heidrich G, Dullin C, Buchhorn G, and Grabbe E:    Preclinical evaluation by flat-panel detector-based volumetric CT    versus MRI of intervertebral spacers implanted in a porcine model.    Spine J 2007; 7(3): p. 360-7. PMID: 17482122.-   28. Ernstberger T, Heidrich G, Schultz W, and Grabbe E: Implant    detectibility of intervertebral disc spacers in post fusion MRI:    evaluation of the MRI scan quality by using a scoring system—an in    vitro study. Neuroradiology 2007; 49(2): p. 103-9. PMID: 17086407.-   29. Starcukova J, Starcuk Z, Jr., Hubalkova H, and Linetskiy I:    Magnetic susceptibility and electrical conductivity of metallic    dental materials and their impact on MR imaging artifacts. Dent    Mater 2008; 24(6): p. 715-23. PMID: 17884157.-   30. Ernstberger T, Buchhorn G, and Heidrich G: Artifacts in spine    magnetic resonance imaging due to different intervertebral test    spacers: an in vitro evaluation of magnesium versus titanium and    carbon-fiber-reinforced polymers as biomaterials. Neuroradiology    2009; 51(8): p. 525-9. PMCID: PMC3085728.-   31. Ernstberger T, Buchhorn G, and Heidrich G: Magnetic resonance    imaging evaluation of intervertebral test spacers: an experimental    comparison of magnesium versus titanium and carbon fiber reinforced    polymers as biomaterials. Ir J Med Sci 2010; 179(1): p. 107-11.    PMCID: PMC3128752.-   32. Heyse T J, Chong le R, Davis J, Boettner F, Haas S B, and Potter    H G: MRI analysis of the component-bone interface after TKA. Knee    2012; 19(4): p. 290-4. PMID: 21741843.-   33. Pauchard Y, Smith M R, and Mintchev M P: Improving geometric    accuracy in the presence of susceptibility difference artifacts    produced by metallic implants in magnetic resonance imaging. IEEE    Trans Med Imaging 2005; 24(10): p. 1387-99. PMID: 16229424.-   34. Jin Z, Xia L, and Du Y P: Reduction of artifacts in    susceptibility-weighted MR venography of the brain. J Magn Reson    Imaging 2008; 28(2): p. 327-33. PMCID: PMC2782378.-   35. Volz S, Hattingen E, Preibisch C, Gasser T, and Deichmann R:    Reduction of susceptibility-induced signal losses in    multi-gradient-echo images: application to improved visualization of    the subthalamic nucleus. Neuroimage 2009; 45(4): p. 1135-43. PMID:    19349229.-   36. Olsen R V, Munk P L, Lee M J, Janzen D L, MacKay A L, Xiang Q S,    and Masri B: Metal artifact reduction sequence: early clinical    applications. Radiographics 2000; 20(3): p. 699-712. PMID: 10835123.-   37. Kolind S H, MacKay A L, Munk P L, and Xiang Q S: Quantitative    evaluation of metal artifact reduction techniques. J Magn Reson    Imaging 2004; 20(3): p. 487-95. PMID: 15332257.-   38. Ramos-Cabrer P, van Duynhoven J P, Van der Toorn A, and Nicolay    K: MRI of hip prostheses using single-point methods: in vitro    studies towards the artifact-free imaging of individuals with metal    implants. Magn Reson Imaging 2004; 22(8): p. 1097-103. PMID:    15527996.-   39. Toms A P, Smith-Bateman C, Malcolm P N, Cahir J, and Graves M:    Optimization of metal artefact reduction (MAR) sequences for MRI of    total hip prostheses. Clin Radiol 2010; 65(6): p. 447-52. PMID:    20451011.-   40. Koch K M, Brau A C, Chen W, Gold G E, Hargreaves B A, Koff M,    McKinnon G C, Potter H G, and King K F: Imaging near metal with a    MAVRIC-SEMAC hybrid. Magn Reson Med 2011; 65(1): p. 71-82. PMID:    20981709.-   41. Sutter R, Ulbrich E J, Jellus V, Nittka M, and Pfirrmann C W:    Reduction of metal artifacts in patients with total hip arthroplasty    with slice-encoding metal artifact correction and view-angle tilting    MR imaging. Radiology 2012; 265(1): p. 204-14. PMID: 22923720.-   42. Lee Y H, Lim D, Kim E, Kim S, Song H T, and Suh J S: Usefulness    of slice encoding for metal artifact correction (SEMAC) for reducing    metallic artifacts in 3-T MRI. Magn Reson Imaging 2013; [Epub ahead    of print]. PMID: 23290476.-   43. Elison J M, Leggitt V L, Thomson M, Oyoyo U, and Wycliffe N D:    Influence of common orthodontic appliances on the diagnostic quality    of cranial magnetic resonance images. Am J Orthod Dentofacial Orthop    2008; 134(4): p. 563-72. PMID: 18929275.-   44. Cox R J, Kau C H, and Rasche V: Three-dimensional ultrashort    echo magnetic resonance imaging of orthodontic appliances in the    natural dentition. Am J Orthod Dentofacial Orthop 2012; 142(4): p.    552-61. PMID: 22999679.-   45. Cullity B D and Graham C D: Introduction to Magnetic Materials.    2nd ed. 2009: Wiley.-   46. Wen Z, Fahrig R, Williams S T, and Pelc N J: Shimming with    permanent magnets for the x-ray detector in a hybrid x-ray/MR    system. Med Phys 2008; 35(9): p. 3895-902. PMCID: PMC2673662.-   47. ACR: Phantom Test Guidance. Available from:    http://www.acr.org/˜/media/ACR/Documents/Accreditation/MRI/LargePhantomGuidance.pdf.-   48. Phelan A, Petocz P, Walsh W, and Darendeliler M A: The    force-distance properties of attracting magnetic attachments for    tooth movement in combination with clear sequential aligners. Aust    Orthod J 2012; 28(2): p. 159-69. PMID: 23304964.-   49. TDK: Magnet Design Data. Available from:    http://www.tdk.co.jp/magnet_e/e371.pdf.-   50. Gholipour A, Kehtarnavaz N, Scherrer B, and Warfield S K: On the    accuracy of unwarping techniques for the correction of    susceptibility-induced geometric distortion in magnetic resonance    Echo-planar images. Conf Proc IEEE Eng Med Biol Soc 2011; 2011: p.    6997-7000. PMID: 22255949.-   51. Morelli J, Porter D, Ai F, Gerdes C, Saettele M, Feiweier T,    Padua A, Dix J, Marra M, Rangaswamy R, and Runge V: Clinical    evaluation of single-shot and readout-segmented diffusion-weighted    imaging in stroke patients at 3 T. Acta Radiol 2013; [Epub ahead of    print]. PMID: 23319722.-   52. Adam C J, Askin G N, and Pearcy M J: Gravity-induced torque and    intravertebral rotation in idiopathic scoliosis. Spine (Phila    Pa. 1976) 2008; 33(2): p. E30-7. PMID: 18197088.-   53. Palinkas M, Nassar M S, Cecilio F A, Siessere S, Semprini M,    Machado-de-Sousa J P, Hallak J E, and Regalo S C: Age and gender    influence on maximal bite force and masticatory muscles thickness.    Arch Oral Biol 2010; 55(10): p. 797-802. PMID: 20667521.-   54. Kagetsu N J and Litt A W: Important considerations in    measurement of attractive force on metallic implants in MR imagers.    Radiology 1991; 179(2): p. 505-8. PMID: 2014301.-   55. Lopic N, Jelen A, Vrtnik S, Jaglicic Z, Wencka M, Starc R, Blinc    A, and Dolinsek J: Quantitative determination of magnetic force on a    coronary stent in MRI. J Magn Reson Imaging 2013; 37(2): p. 391-7.    PMID: 23125054.-   56. Bondemark L, Kurol J, and Wennberg A: Orthodontic rare earth    magnets—in vitro assessment of cytotoxicity. Br J Orthod 1994;    21(4): p. 335-41. PMID: 7857892.-   57. Donohue V E, McDonald F, and Evans R: In vitro cytotoxicity    testing of neodymium-iron-boron magnets. J Appl Biomater 1995;    6(1): p. 69-74. PMID: 7703540.-   58. Bondemark L: Orthodontic magnets. A study of force and field    pattern, biocompatibility and clinical effects. Swed Dent J Suppl    1994; 99: p. 1-148. PMID: 7801229.-   59. Boeckler A F, Morton D, Ehring C, and Setz J M: Mechanical    properties of magnetic attachments for removable prostheses on teeth    and implants. J Prosthodont 2008; 17(8): p. 608-15. PMID: 18761583.

1. An apparatus comprising: an arch-shaped body configured to be worninside of a user's mouth such that the arch-shaped body follows acontour of at least some of the user's teeth; where the arch-shaped bodycomprises one or more sidewalls and a biting member, the biting memberconfigured to be disposed between the user's mandibular and maxillaryteeth, the one or more sidewalls angularly disposed relative to thebiting member and configured to be coupled to a plurality of memberscomprising magnetically permeable material.
 2. The apparatus of claim 1,where the magnetically permeable material comprises one or morepermanent magnets.
 3. The apparatus of claim 2, where the one or morepermanent magnets comprise a plurality of permanent magnets.
 4. Theapparatus of claim 1, further comprising a handle configured to protrudefrom the user's mouth.
 5. An apparatus comprising: an arch-shaped bodyconfigured to be worn outside of a user's mouth such that thearch-shaped body follows a contour of the user's face; and where thearch-shaped body comprises one or more sidewalls configured to becoupled to a plurality of members comprising magnetically permeablematerial.
 6. The apparatus of claim 5, where the magnetically permeablematerial comprises one or more permanent magnets.
 7. The apparatus ofclaim 6, where the one or more permanent magnets comprise a plurality ofpermanent magnets.
 8. The apparatus of claim 5, where the arch-shapedbody does not obstruct an airway of the user when worn outside theuser's mouth.
 9. The apparatus of claim 5, where the plurality ofmembers are substantially adjacent the user's maxilla and mandible,partially or substantially above the user's maxilla, or partially orsubstantially below the user's mandible.
 10. The apparatus of claim 5,further comprising a forehead support, where the forehead support isconfigured such that it follows a contour of the user's forehead. 11.The apparatus of claim 10, where the forehead support comprises aplurality of openings.
 12. The apparatus of claim 10, further comprisinga frame disposed at least in part between the arch-shaped body and theforehead support.
 13. The apparatus of claim 12, where the frame issubstantially rigid.
 14. The apparatus of claim 12 further comprising aplurality of nut-and screw fasteners.
 15. The apparatus of claim 12,further comprising one or more straps configured to secure any of thefollowing to the front of the user's head: the arch-shaped body, theforehead support, the frame.
 16. The apparatus of claim 5, where atleast one of the one or more sidewalls comprises a curved surface, andthe apparatus is configured to be worn by a user such that normalvectors along the surface lie substantially in a plane perpendicular toa magnetic field of a magnetic resonance imaging scanner.
 17. Theapparatus of claim 5, where at least some of the plurality of memberscomprise ferromagnetic material.
 18. The apparatus of claim 5, furthercomprising a layer of material configured to be coupled to the at leastone of the one or more sidewalls such that the layer of materialoverlies each of the plurality of members.
 19. The apparatus of claim 5,where the plurality of members is configured to partially restore lossesin magnetic field homogeneity caused by non-biological materials withinthe user's mouth during magnetic resonance imaging.
 20. The apparatus ofclaim 5, where a total magnetic moment generated by the plurality ofmembers is substantially equal but opposite to the magnetic momentinduced by non-biological materials within the user's mouth duringmagnetic resonance imaging.
 21. The apparatus of claim 19, where thenon-biological materials within the user's mouth comprise dental braces.22. An apparatus comprising: a first arch-shaped body configured to beworn inside a user's mouth such that the arch-shaped body follows acontour of at least some of the user's teeth; where the firstarch-shaped body comprises a one or more sidewalls and a biting member,the biting member configured to be disposed between the user'smandibular and maxillary teeth, the one or more sidewalls angularlydisposed relative to the biting member and configured to be coupled to aplurality of members comprising magnetically permeable material; and asecond arch-shaped body configured to be worn outside a user's mouthsuch that the second arch-shaped body follows a contour of the user'sface; where the arch-shaped body comprises one or more sidewallsconfigured to be coupled to a plurality of members comprisingmagnetically permeable material.
 23. The apparatus of claim 22, where atleast some of the plurality of members comprise ferromagnetic material.24. The apparatus of claim 23, where the magnetically permeable materialcomprises one or more permanent magnets.
 25. The apparatus of claim 24,where the one or more permanent magnets comprise a plurality ofpermanent magnets.
 26. The apparatus of claim 23, where at least one ofthe one or more sidewalls of any of the first and second arch-shapedbody are configured to generate magnetic moments different than at leastone other of the one or more sidewalls of any of the first and secondarch-shaped body.